1. Field of the Invention
The present invention relates to an apparatus and method for concurrently reducing the cyanide and heavy metal content of aqueous solutions. Broadly, the apparatus and method are based on electrolytic techniques and include an electrochemical cell having separate anode and cathode compartments.
There is considerable and growing concern over pollution of the nation's waterways with various contaminants such as cyanide, mercury and various heavy metals. Mercury is known to be a neuro-poison and is especially dangerous in the alkyl mercury form in which it is often found in water and aquatic life. Cadmium in river water has been identified as the cause of a painful disease (itai itai). Lead and other heavy metals are suspected of being dangerous pollutants in our waters. Cyanide is toxic to many forms of life and, obviously, is an undesirable pollutant.
Many of these pollutants enter our waters from industrial sources, such as metal finishing plants and from mining sources. Recent steps to limit the pollution from these sources have been relatively effective. However, some reduced but significant quantities of these pollutants will continue to come from industries and mines unless treatment specifically directed toward their removal is given.
Contamination also may arise from "natural" sources, e.g., mercury at levels to cause concern has been found in lakes where little, if any, human activity has occurred. To clean up these waters, the only course open is to treat them to remove the specific contaminants.
2. Prior Art
Various methods have been proposed or are reported to be available and in use for removal of cyanide and various heavy metals from water.
A method of decreasing the amount of cyanide ions in a solution is described in U.S. Pat. No. 3,764,497. The method comprises passing a current through an electrolytic cell containing (a) at least one positive and one negative electrode, (b) an electrolyte containing cyanide ions, and (c) a bed of nonconductive particles. The particles are distributed by the flow of the electrolyte through the bed of particles. The patent also discloses that the copper content of the electrolyte is decreased.
Another method of decreasing the cyanide content of a solution is described in U.S. Pat. No. 3,764,498. The method comprises passing the solution through an electrolytic cell containing at least one positive and one negative electrode and a bed of particles having a certain specified bed porosity. The patent discloses that the particles may be either conductive, nonconductive or semiconductive. Suitable conductive materials disclosed include copper, silver, zinc, graphite and the like. The patent also discloses that some of the copper in the solution was plated on the cathode due to the electrochemical reduction of copper to the metal.
In U.S. Pat. No. 3,457,152 there is disclosed an electrolytic apparatus and process for removing trace metals from an aqueous solution. Broadly, the process comprises subjecting the solution to a direct electric current in the presence of at least one anode and a lead shot cathode.
A method of decreasing the metallic content of liquids also is found in U.S. Pat. No. 3,755,114. The method described therein comprises passing an electric current through a solution containing metallic materials, which solution is contained as the electrolyte in a cell. The cell has at least one positive and one negative electrode between which the current is passed. The electrolyte also contains a bed of particles distributed therein, such that the porosity of the bed is within certain defined limits.
More recently, in U.S. Pat. No. 3,764,499 there is disclosed a process for the removal of both cyanide and metals from a waste water. The method comprises introducing the waste water into a cell having a first major electrode chamber which is filled by granular or fibrous electric conductors and a major electrode. The first major electrode chamber is separated from a second electrode chamber of opposite polarity by an ion exchange membrane. A potential is applied across the cell to effect electrolytic reaction with decomposition of the contaminants in the first major electrode chamber and with migration of the residual contaminants through the membrane into the second electrode chamber for further electrolytic reaction with decomposition of the residual contaminants. The Patentee suggests the use of various materials for the granular or fibrous electric conductors including graphite, metal particles, platinum particles and titanium particles.
The foregoing proposed and currently commercially available methods for metal removal, e.g., chemical precipitation, reverse osmosis, ion exchange, and absorption, are generally costly, require elaborate equipment, are specific for only a few metals, or present serious waste disposal problems. In addition, they are, in general, not able to attain the very low effluent metal levels contemplated in prospective future governmental regulations dealing with water contamination. Certain of these methods merely substitute one undesirable metal ion in place of the metal ion being removed. Clearly, therefore, there is need for an effective economical method of removing cyanide and metal ions from an aqueous solution such as an industrial waste water.
Other patents considered in the preparation of this application are U.S. Pat. Nos. 2,520,703; 3,730,864; 3,728,238; 2,773,025; 3,515,664; 3,616,356; 3,679,557; and 3,766,034.